Rather than report "undefined symbol: ", give more informative message
about the object file that defines the discarded section.
In particular, PR41133, if the section is a discarded COMDAT, print the
section group signature and the object file with the prevailing
definition. This is useful to track down some ODR issues.
We need to
- add uint32_t DiscardedSecIdx to Undefined for this feature.
- make ComdatGroups public and change its type to DenseMap<CachedHashStringRef, const InputFile *>